1. Field of the Invention
The present invention relates to a device for eliminating or alleviating voltage unevenness among unit cells connected in series to form a combination battery. The unit cells are rechargeable battery cells such as lithium batteries, and the combination battery is used as an electric power source for driving an electric vehicle or a hybrid electric vehicle.
2. Description of Related Art
A vehicle powered by an electric power source such as a battery is recently becoming a hot subject form an ecology standpoint. However, a battery has not been sufficiently developed for economically and efficiently driving the vehicle. A hybrid electric vehicle (HEV) powered by both a battery and an engine is one of the solutions for compensating insufficiency of the battery. The HEV is driven by an engine when the engine operates in relatively high efficiency with less harmful exhaust emissions, while it is driven by the battery when the vehicle is starting and accelerating. Therefore, harmful exhaust emissions can be reduced with a help of battery having a relatively small capacity. An electric motor, a battery and other electric components have to be mounted on the HEV in addition to a conventional engine, and therefore the weight of the HEV tends to become much heavier than a conventional vehicle. Therefore, it is important to make the battery compact and efficient. For this purpose, a rechargeable lithium battery, in place of a conventional battery such as a lead-acid, nickel-cadmium or nickel-hydrogen battery, is attracting a keen interest. An energy density of the lithium battery is about four times of a lead-acid battery having a same capacity and about two times of a nickel-hydrogen battery. However, the rechargeable lithium battery is not robust against over-charge or over-discharge and has to be used within a predetermined voltage range to prevent excessive heating or dissolution of materials used therein. Therefore, the lithium battery is usually charged under a constant voltage or used together with a protection device for limiting a voltage range. In addition, many unit cells have to be connected in series to obtain a high voltage for driving a motor of the HEV. For example, 150 cells of the lead-acid battery each having 2 volts are connected in series for obtaining a terminal voltage of 300 volts, or 250 cells of the nickel-hydrogen battery are connected in series. In case of a lithium battery cell having 3.6 volts, 80 cells have to be connected in series. In a conventional combination battery using many cells connected in series, its terminal voltage is always detected, and it is charged and discharged based on the detected terminal voltage. For example, the terminal voltage is controlled between 270-360 volts in case of a lead-acid combination battery consisting of 150 cells each having 1.8-2.4 volts.
When the charging and discharging of a combination battery are controlled based on a total terminal voltage, there is a problem that voltage unevenness among individual cells cannot be avoided. That is, voltages of individual cells are not always equal to one another due to difference of individual cells or temperature difference. If the combination battery is charged up to a higher limit of the terminal voltage, some of the individual cells may be over-charged even an average voltage for all the cells is controlled within the voltage limit. If the combination battery is discharged up to a lower limit of the terminal voltage, some of the individual cells may be over-discharged. In case of the lead-acid, nickel-cadmium or nickel-hydrogen batteries, the combination battery is not heavily damaged even if over-charge or over-discharge of the individual cells occur, though its capacity or performance may be reduced somewhat. Therefore, such combination batteries are usually controlled based on the total terminal voltage.
However, in case of the lithium battery, such over-charge or over-discharge of the individual cells may be fatal. It should be definitely avoided. To cope with this problem, JP-A-2-136445, for example, proposes to detect an individual cell showing the highest voltage to stop charging before it exceeds a predetermined voltage, and to detect an individual cell showing the lowest voltage to terminate discharging before it falls below a predetermined voltage. According to the proposed control, all the cells constituting the combination battery can be protected from the over-charge or over-discharge. However, there is a disadvantage that total capacity of the combination battery is sacrificed because the charging is limited by the cell having the highest voltage and the discharging is limited by the cell having the lowest voltage.
Another solution to overcome the voltage unevenness is proposed in JP-A-6-253463, for example. In this proposal, a bypass circuit consisting of a resistance and a switch is connected to each cell in parallel. A cell having a higher voltage is discharged through the bypass circuit, or charging current is bypassed through the bypass circuit during a charging period. In this manner, voltage unevenness among individual cells is alleviated. Yet another solution is disclosed in JP-A-5-64377 and JP-A-8-213055. The degree of the charging in each cell is detected and the cell which has been fully charged is discharged through a bypass circuit, and all the cells are brought to a fully charged state in this manner. Thus, the voltage unevenness is eliminated.
However, to realize the control proposed by those disclosures, it is required to provide a complex and expensive control device as exemplified in FIG. 19. Discharge circuits 603, each consisting of a resistance 603a and a switch 603b, are connected to a combination battery 601 consisting of unit cells 602(1)-602(n) in parallel. Each switch 603b is controlled by a photo-coupler 610 (or an isolation amplifier) which in turn is controlled by a decoder 609. The voltage of each cell 602(1)-602(n) is detected by a voltage detector 604, and the detected voltage is fed to a multiplexer 605. The output of the multiplexer 605 is converted from an analog signal to a digital signal by an A/D converter 606. Then, the digital signal is processed by a microprocessor unit (MPU) 607 which communicates with a memory 608 and a controller for controlling vehicle operation. The MPU 607 controls the decoder 609. As shown in FIG. 19, a complex and expensive control system is necessary to realize the proposed voltage unevenness control. Especially, it has to be noted that a large number of components corresponding to the number of the unit cells are required in the system. Moreover, the MPU becomes expensive because it has to process many signals corresponding to each cell.
Another example of the voltage unevenness control device is disclosed in JP-A-8-55643, in which a comparator is used to compare the voltages of a pair of neighboring individual cells. One cell having a higher voltage than the other is discharged through a discharge circuit including a complementary transistors. A large number of control modules, each consisting of a voltage divider resistance, the comparator and the complementary transistors, are required in this device. If the number of unit cells constituting the combination battery is 80, 79 control modules are necessary. This makes the control device expensive. Moreover, since the voltage unevenness between a pair of neighboring cells is controlled by a control module, there is a problem that control errors among a number of modules may be accumulated and a precise control as a whole may not be attained.